This invention relates to biological filter structures.
Volatile organic compounds (VOCs) of various kinds are widely used in many industrial processes. Typical VOCs include acetone, benzene, toluene, xylene and methyl ethyl ketone (MEK). Disposal of VOCs or substances containing VOCs represents a significant problem for many industries.
A known technique for disposal and/or destruction of VOCs from certain sources is to allow any volatilizable compounds to evaporate at ambient temperature into an effectively unconfined space. Since the VOCs evaporate at an uncontrolled rate, and the space into which the VOCs evaporate is effectively unconfined, the concentration of VOCs in the atmosphere in the vicinity of the residue material does not normally become bothersome, but this practice nevertheless contributes to atmospheric pollution.
U.S. Pat. No. 5,518,920 discloses apparatus for capturing air that contains VOCs in the vapor phase and for processing the air in order to convert the VOCs to a nontoxic form. In particular, U.S. Pat. No. 5,518,920 discloses a biological filter unit comprising a housing containing biologically active filter material. The filter material includes plant compost, which is typically a mixture of various types of plant debris, such as leaves, grass and wood, and contains microorganisms (fungi and bacteria) that have an affinity for VOCs and consume VOCs as nutrients. The compost may also contain micronutrients such as nitrogen and potassium. The housing has an inlet opening at the bottom for supply of feed air and an exhaust opening at the top. Feed air containing VOCs is supplied to the inlet opening and flows upwards through the filter material. The microorganisms capture and consume the VOCs, and accordingly, under appropriate operating conditions, the concentration of VOCs in the exhaust air is substantially less than the concentration of VOCs in the feed air.
In order to provide the filter unit with sufficient resilience to withstand heavy loadings of VOCs, it is necessary that the filter unit should contain a large population of microorganisms, which in turn requires a large quantity of compost. In the filter unit disclosed in U.S. Pat. No. 5,518,920, the compost is supported in generally horizontal beds. Cost considerations dictate that the number of beds be kept to a minimum, and consequently in order to provide the required quantity of compost, the beds are fairly thick. Specifically, the beds may have a depth of up to about 36 inches.
It is believed that a major part of the biological activity of a biofilter takes place at the surface of the mass of filter material, where the biomatrix is exposed to the incoming pollutant-laden air. Accordingly, it is desirable to maximize the ratio of the surface area of the filter bed to the volume of compost material. A possible disadvantage of the biological filter unit disclosed in U.S. Pat. No. 5,518,920 is that the surface area of the compost beds relative to the volume of the beds is rather small. Further possible disadvantages are that a thick bed of biofilter material may result in a rather large back pressure, requiring use of a large amount of energy to maintain the flow of air through the filter bed, channeling of the air flow through the filter bed, and compaction.
U.S. Pat. No. 4,203,935 discloses a filter media for scrubbers. The filter media is composed of structured packing balls, which are generally spherical bodies having a latticework mantle composed of an equatorial ring from which arcuate bars extend, each bar terminating in a polar region of the spherical body. The structured packing ball is designed to maximize the surface area of the latticework and to provide maximum capture of liquid droplets for effective scrubbing of air passing through the filter media against the liquid droplets.
In accordance with a first aspect of the invention there is provided an article of manufacture comprising a support element of generally ellipsoidal configuration, the support element having two opposite polar regions and including a medial ring intermediate the two opposite polar regions and a plurality of arcuate ribs extending from the medial ring to each polar region, adjacent arcuate ribs being angularly spaced to provide access to an interior space of the support element, and a cohesive body of biologically active material surrounding the support element and penetrating the interior space of the support element.
In accordance with a second aspect of the invention there is provided a method of manufacturing a biologically active filter matrix, comprising providing biologically active material in particulate form, providing a multiplicity of support elements of generally ellipsoidal configuration, each support element having two opposite polar regions and including a medial ring intermediate the two opposite polar regions and a plurality of arcuate ribs extending from the medial ring to each polar region, adjacent arcuate ribs being angularly spaced to provide access to an interior space of the support element, and intimately mixing the support elements with the biologically active material, whereby the biologically active material penetrates the interior spaces of the support elements and is divided into discrete bodies that surround the support elements respectively.
In accordance with a third aspect of the invention there is provided a biologically active filter, comprising a housing, a multiplicity of discrete biologically active filter elements in the housing, each biologically active filter element being substantially ellipsoidal in configuration, being dimensionally stable, having a maximum rectilinear dimension in a range from about 1 cm to about 3 cm, and being composed of a cohesive body of biologically active material.